The present invention relates to semiconductor processing and, more particularly, to thermal reactors for chemical vapor deposition, thermal annealing, and other procedures requiring high temperature processing. A major objective of the present invention is to provide an improved thermal reactor for semiconductor processing with relatively uniform reactant gas flow at non-ambient pressures.
Recent technological progress is closely identified with the increasing miniaturization of electronic circuits made possible by advances in semiconductor processing. Certain advanced processing techniques require exposing a semiconductor structure to a reactant gas under carefully controlled conditions of elevated temperature, sub-ambient pressures and uniform reactant gas flow. Examples of such processes include low-pressure chemical vapor deposition, reduced-pressure chemical vapor deposition, and selective epitaxial deposition. Of particular concern is the uniformity of temperature and gas flow to ensure uniform results, e.g., deposition thickness, across a wafer.
A reactor system for implementing ambient and sub-ambient pressure thermal reactions typically includes a reaction chamber, a gas source, an exhaust system, a heat source, and a coolant source. The reaction chamber provides a controlled environment for the desired reaction. The gas source provides the purging and reactant gases, while the exhaust system removes spent gases and maintains the desired sub-ambient pressures. The heating source, which can be an array of infrared lamps or an inductive source, generally transmits energy through the chamber wall to heat the wafer. Generally, the wafer is mounted on a support structure, which can serve as a susceptor to absorb energy transmitted into the chamber and convey the resulting heat to the wafer being processed. In addition, the support can rotate the wafer within the chamber to minimize the effect of spatial anomalies within the chamber. Coolant is applied to the outer surface of the chamber to minimize thermal expansion and distortion of the chamber during deposition, to minimize deposition on the chamber wall, and to assist cooling after deposition.
Quartz is the material of choice for the reaction chamber wall. Its high melting point and low thermal coefficient of expansion tolerate the high temperatures, e.g., over 1100.degree. C., used in some chemical vapor deposition (CVD) reactions. A transparent dielectric, quartz is compatible with both infrared and inductive heating sources, and it facilitates the dissipation of heat after deposition is completed. Furthermore, quartz can be obtained in very pure form, e.g., fused silica, minimizing its role as a source of contamination in a thermal reaction. Quartz is herein defined as a natural or synthetic glass consisting of at least 90% silicon dioxide.
Reaction chamber walls are typically cylindrical, either over the entire chamber length, or over a substantial portion of the chamber length, e.g., as in a bell jar chamber. Cylindrical vessels are the vessels of choice for low pressure applications since they distribute stress due to pressure differential uniformly and do so in spite of variations in wall thickness. The even distribution of stress minimizes the probability of breakage.
On the other hand, the cylindrical geometry prevents a uniform reactant gas flow at a wafer surface. The wafer surface is flat so that, when symmetrically positioned within a cylindrical chamber, the wafer edges furthest from the longitudinal axis of the chamber are closer to the chamber wall than is the wafer center. Thus, a greater reactant gas volume is provided over the wafer center than is provided over the transverse edges of the wafer. This invites uneven depositions, which will impair the yield of integrated circuits from a wafer. While uneven flow is not a problem for diffusion-driven depositions, which can occur at very low pressures of about 1 Torr or less, it is a problem for flow-driven deposition reactions, which are far more prevalent.
The nonuniformity imposed by cylindrical geometry can be minimized using a large radius of curvature. For example, the AMC-7810/11 Cylindrical Epitaxial Reactor, manufactured by Applied Materials, Inc., is designed to process concurrently circumferential rows of wafers arranged around a multi-faceted barrel. On the scale of the individual wafers, the cylindrical chamber wall can be relatively flat, and thus reactant gas flow over the wafer can be relatively uniform. However, as larger wafers are to be processed, scaling such a reactor design is problematic. The volume of hardware required increases much faster than linearly with wafer diameter; the larger volumes and masses involved in larger systems require longer heating and cooling times, impeding throughput. The problem with throughput is further aggravated where, for experimental or custom objectives, a single wafer is to be processed since there is virtually no processing time saved when utilizing such a chamber below capacity.
As the foregoing illustrates, there are conflicting design objectives inherent in chamber design. Reduced-pressure processing indicates cylindrical geometries and small radii of curvature. Uniform reactant flow indicates large radii of curvature, either flat surfaces or very large diameter cylinders. In addition, chamber wall curvature can distort radiant energy transmitted therethrough, thus causing a wafer to heat unevenly. Uneven heating can result in uneven deposition and crystal slippage. A chamber wall with flat surfaces would alleviate this problem. Ambient pressure thermal reactors exist which use rectangular quartz chambers to provide uniform reactant gas flow across a wafer. However, these are not reduced pressure or low pressure capable. The pressure differential across a flat surface would cause localized stress and subject it to breakage.
Stressing of flat and other non-cylindrical walls due to pressure differential can be addressed by using greater wall thicknesses. However, thick walls provide too much thermal insulation. The effectiveness of the external coolant, typically air, in reducing chamber wall temperatures would be reduced, leading to increased chemical deposition on the inner surfaces of the wall. Furthermore, the hotter inner surfaces would tend to expand more rapidly than the outer surfaces, causing the chamber wall to crack. Additional design objectives result in conflicting preferences between thin chamber walls, for both reduced thermal stresses and wall deposition versus thicker walls to reduce pressure-caused stresses.
The requirement for uniformity also applies to chamber cooling. As indicated above, a coolant fluid is passed across the outer surface of the chamber wall to minimize depositions on the chamber wall and distortions due to thermal expansion. Generally, coolant fluid is applied through nozzles near or behind the heat source. Coolant flow generally is not controlled at the chamber wall surface so that eddies and other flow irregularities can result. This can cause differential cooling across the chamber wall, resulting in local depositions and uneven stresses which could crack the chamber wall.
While pressure differentials are most commonly encountered in sub-ambient pressure procedures, they also apply to procedures conducted at above ambient pressure. For example, many procedures considered as ambient pressure procedures are so only when the ambient pressure is about 760 Torr. A laboratory in an elevated location, e.g., Denver, might need to pressure a reaction vessel to carry out such a procedure. The conflicting design objectives applicable to sub-ambient pressure processing would also apply to such cases of above-ambient pressure processing.
What is needed is a thermal reactor system for semiconductor processing which provides for processing with elevated temperatures, non-ambient pressures and uniform reactant flows. In addition, heating and cooling uniformities should be provided.